In recent years, computers and networks have advanced remarkably, and many kinds of digital data such as text data, image data, audio data, and the like are handled on the computers and networks.
From such digital data (digital information), a copy of data with an equivalent quality can be easily formed. For this reason, to protect the copyrights of data, copyright information, user information, and the like are often embedded as digital watermarks in digital data such as image data, audio data, and the like, which are to undergo copyright protection.
Note that “digital watermarking” is a technique for embedding another information in secrecy in digital data to be processed by a predetermined process.
Therefore, by extracting digital watermark information from digital data embedded with the digital watermark, the copyright information, user information, identification information, and the like of that data can be obtained, and illicit copies can be traced.
As one of conditions required for such digital watermarking, digital watermark information embedded in digital data must be imperceptible; digital watermark information must be able to be embedded with least quality deterioration of source digital data (first condition).
For example, when digital data to be processed is image data, general digital watermarking often adopts schemes that utilize given features of the image, e.g., a scheme for embedding digital watermark information in a high-frequency region by exploiting the human visual characteristics, a scheme of extracting a portion with a large change such as an edge or the like, and embedding digital watermark information around that extracted portion, and so forth.
More specifically, digital watermarking can be roughly classified into two methods, e.g., a method of embedding in the spatial domain and a scheme of embedding in the frequency domain.
Examples of the method of embedding digital watermark information in the spatial domain include schemes described in [W. Bender, D. Gruhl, & N. Morimoto, “Techniques for Data Hiding”, Proceedings of the SPIE, San Jose Calif., USA, February 1995], [G. B. Rhoads & W. Linn, “Steganography methods employing embedded calibration data”, U.S. Pat. No. 5,636,292], and the like, which employ patchworks.
Examples of the method of embedding digital watermark information in the frequency domain include a scheme [Nakamura, Ogawa, & Takashima, “A Method of Watermarking in Frequency Domain for Protecting Copyright of Digital Image”, SCIS' 97-26A, January 1997], which exploits discrete cosine transformation, a scheme [Onishi, Oka, & Matsui, “A Watermarking Scheme for Image Data by PN Sequence”, SCIS' 97-26D, January 1997] which exploits discrete Fourier transformation, and schemes [Ishizuka, Sakai, & Sakurai, “Experimental Evaluation of Steganography Using Wavelet Transform”, SCIS' 97-26D, January 1997] and [Inoue, Miyazaki, Yamamoto, & Katsura, “A Digital Watermark Technique based on the Wavelet Transform and its Robustness against Image Compression and Transformation”, SCIS' 98-3.2.A, February 1998] last two of which exploit discrete wavelet transformation, and the like.
As copyright protection systems that exploit digital watermarking, systems described in [Iwamura, Sakurai, & Imai, “Proposal of Blind Digital Watermarking”, IEICE Transactions ISEC97-35], [Iwamura, Sakurai, & Imai, “Secure Digital Watermarking System for Secondary Distribution”, SCIS' 98-10.2F], and the like have been proposed. These systems adopt a scheme that does not use a characteristic of an image to embed a digital watermark in encrypted digital contents (image data). Therefore, watermarked image using this system has bad quality.
As the second condition (robustness) required for digital watermarking in addition to the aforementioned first condition (quality), digital watermark information embedded in digital data must remain undisturbed, i.e., embedded digital watermark information must never be lost by editing or attacks such as data compression, a filter process, and the like. Furthermore, the third condition (information size) required for digital watermarking is that the information size of digital watermark information to be embedded must be able to be selected in accordance with purposes of use.
The first to third conditions required for digital watermarking normally have a trade-off relationship. For example, upon implementing robust digital watermarking, relatively large quality deterioration occurs, and the information size of a digital watermark to be embedded becomes small.
Hence, digital watermarking is used in correspondence with various purposes of use due to the presence of such trade-off.
For example, when digital data to be processed is image data which is to be displayed on the screen of a monitor of a personal computer (PC), since the monitor has a low resolution, and the image data can be easily altered by image edit software or the like by the PC, digital watermarking that attaches an importance on robustness rather than quality (image quality) is used for such image data.
In recent years, printers can attain high image quality, and can output images with quality called photo quality. An output image at that time is a printout, and it is not easy to alter that image. Therefore, in such case, digital watermarking that attaches an importance on quality (image quality) is used to have only robustness against print and scan processes.
However, since the aforementioned conventional copyright protection systems that exploit digital watermarking adopt a scheme that does not use a characteristic of an image to embed a digital watermark in encrypted digital contents (image data), high quality (image quality) of the image cannot be assured.
Also, since the first to third conditions required for digital watermarking have a trade-off relationship, as described above, digital watermark processing software programs are present as standalone software programs for respective purposes or as a library of a group of these programs, but no comprehensive system for all of such programs is available.